1. Field of the Invention
The present invention generally relates to a wavelength stabilized light source, and more particularly to a wavelength stabilized light source which is suitable for a light source for use in coherent optical communications.
2. Background Art
Currently, optical communications are widely used. In optical communications, it is important to use a light source which emits a light having a stabilized wavelength. Presently, two different kinds of wavelength stabilized light sources have been put to practical use.
FIG. 1A is a block diagram of a first kind of conventional wavelength stabilized light source. A laser diode (semiconductor laser) LD emits a light or laser beam having a wavelength of 1.55 .mu.m. The laser beam emitted from the laser diode LD is split into two beams by a beam splitter BS. One of the split beams enters a transmission line formed by an optical fiber OF, and the other beam passes through a filter FL and is supplied to an opto-electric (photoelectric) converter O/E. The filter FL is designed to pass a light having a wavelength .lambda..sub.0 equal to that of the laser beam to be penetrated in the optical filter OF. The opto-electric converter O/E converts the incident laser beam into a corresponding electric signal, which is supplied to a drive DV. The electric signal changes based on a deviation of the laser beam from the wavelength .lambda..sub.0 of the filter FL. Thus, the driver DV changes a driving current based on the electric signal derived from the opto-electric converter O/E so that the laser diode LD emits the predetermined wavelength of light equal to 1.55 .mu.m.
FIG. 1B is a block diagram of a second conventional wavelength stabilized light source. The driver DV outputs a constant driving current to the laser diode LD, which emits not only light to be entered into the optical fiber OF but also light to be entered into an etalon ET. The etalon ET is disposed at a position where n.lambda..sub.0 =.lambda..sub.0 is satisfied (n is an integer). A light having the wavelength .lambda..sub.0 resonates in the etalon ET and returns to the laser diode LD. Thus, only the laser beam having the wavelength .lambda..sub.0 is increased and emitted toward the optical fiber OF.
However, the wavelength of light provided by the prior art shown in FIG. 1A is based on precision of the filter FL. Similarly, the wavelength of light provided by the prior art shown in FIG. 1B is based on precision of the etalon ET. For these reasons, neither the prior art shown in FIG. 1A nor the prior art shown in FIG. 1B cannot provide light of the wavelength which is absolutely equal to the desired wavelength .lambda..sub.0. It is noted that recently there has been considerable activity in the development of phase modulation or frequency multiplexing in coherent optical communications. In such advanced optical communications, it is essential to use a light source which is capable of emitting light having an absolutely fixed (stabilized) wavelength.
FIG. 2 is a block diagram of a third conventional wavelength stabilized light source, which utilizes an absorption line of atoms (or molecules). The light source shown in FIG. 2 is superior to the prior art shown in FIG. 1A or 1B. Referring to FIG. 2, the illustrated light source is made up of a laser diode (semiconductor laser) 31, a beam splitter 32, a phase modulator 33, an absorption cell 34 in which an NH.sub.3 gas is filled, a photodetector 35, an amplifier 36, a synchronous rectifying circuit 37, an oscillator 38 outputting a frequency fm, a control circuit 39, a laser diode driver 40 and a temperature controller 41. A laser beam emitted from the laser diode 31 is split into two beams by the beam splitter 32. One of the split beams is drawn as a light output of the light source, and the other beam is phase-modulated by the frequency fm through the phase modulator 33. The phase-modulated beam enters the absorption cell 34 which has an NH.sub.3 gas having a resonance wavelength .lambda..sub.0 within a band in the order of 1.5 .mu.m. When the wavelength of the incident beam becomes equal to the wavelength of atoms (molecules) filled in the absorption cell 34, the incident light is absorbed the most greatly in the absorption cell 34. Thus, the photodetector 35 on which light from the absorption cell 34 is irradiated, generates a photodetector output as shown in a graph of FIG. 3 where the horizontal axis represents wavelength of the laser beam emitted from the laser diode 31 and the vertical axis thereof represents D.C. voltage value of the photodetector output. When the light from the absorption cell 34 is attenuated most greatly, the wavelength of the laser beam emitted from the laser diode 31 is equal to the resonance wavelength .lambda..sub.0.
Since the incident light to the absorption cell 34 has been phase-modulated by the frequency fm, the photodetector output includes a signal component (an alternating component), the phase of which is different by 180 degrees on both sides of the resonance wavelength .lambda..sub.0 the absorption cell 34. Particularly, when the wavelength of the incident light is equal to the resonance wavelength .lambda..sub.0, the photodetector output includes a signal component of a frequency twice the modulation frequency fm (that is, a second-harmonic frequency signal of frequency 2fm).
The photodetector output is amplified through the amplifier 36 and supplied to the synchronous rectifying circuit 37, which subjects the amplified photodetector output to a synchronous detection process by the frequency fm generated by the oscillator 38. FIG. 4 is a graph of a synchronous detection output characteristic where the horizontal axis thereof represents wavelength of the laser diode 31 and the vertical axis represents the level of the synchronous detection output (D.C. voltage) derived from the synchronous rectifying circuit 37. When the laser diode 31 emits a light having a wavelength shorter than the resonance wavelength .lambda..sub.0, the polarity of the synchronous detection output is negative. Adversely, when the laser diode 31 emits a light having a wavelength longer than the resonance wavelength .lambda..sub.0, the polarity of the synchronous detection output is positive.
The control circuit 39 drives the laser diode driver 40 so that the synchronous detection output supplied from the synchronous rectifying circuit 37 becomes zero at the resonance wavelength .lambda..sub.0. The laser diode driver 40 controls the driving current to be supplied to the laser diode 31 so that the wavelength of the beam emitted from the laser diode 31 is always equal to the wavelength .lambda..sub.0. It has been confirmed that the Q of the conventional resonance characteristic is approximately 1.times.10.sup.-5 and the fluctuation (stability over a short period) of wavelength .sigma.y(.tau.) is approximately 1.times.10.sup.-11.
It is noted that at present the optimum wavelength of light suitable for coherent optical communications is 1.56 .mu.m in view of efficiency in transmission loss caused in optical fiber. The light output supplied from the beam splitter 32 shown in FIG. 2 is of the wavelength equal to 1.56 .mu.m. Since the wavelength of light is highly stabilized, the conventional light source shown in FIG. 2 can conceivable to be used as a light source in coherent optical communications.
However, the light source shown in FIG. 2 presents the following disadvantage. That is, the degree of light absorption caused in the absorption cell 34 shown in FIG. 2 is very weak (in other words, the concave peak of the resonance pattern is shallow). For this reason, the stability of the light source is not good.